With deployment of small cells, an important challenge is to provide connectivity to the small cell sites. Several operators are considering the fixed access infrastructure with its deep penetration as an important avenue for providing connectivity to the small cell sites. Current fixed access networks are largely based on copper (xDSL, Digital Subscriber Line), but fibre is becoming increasingly common through deployment of fibre-to-the-home/building/curve (FTTH/FTTB/FTTC). This opens up the interest for use of optical technologies for providing small cell connectivity.
There are several scenarios for how the fixed access infrastructure could be exploited for small cell transport in mobile networks. One scenario is based on network sharing on layers 2 and 3, L2/L3. Small cells could for example be backhauled through L2/L3 packet aggregation reusing fixed access systems and where multi-service/multi-operator sharing of the network is implemented on L2/L3. Another possibility is the implementation of multi-service/multi-operator infrastructure sharing on lower layers. This could for example be achieved on the wavelength level. This means that there would be dedicated systems for small cell transport reusing the fixed access infrastructure via e.g. a wavelength overlay. An example of where this could be required is for the case of centralised deployment of small cells based on fronthaul where stringent latency/jitter requirements prohibit the use of traditional fixed access systems (such as Time Division Multiplex-Passive Optical Network, TDM-PON) for fronthaul. Hence, small cell densification is one factor that could drive an increased need for multi-operator/multi-service sharing of the access infrastructure. Also, in a the context of the Networked Society it will become increasingly important for a fibre infrastructure provider to be able to provide wavelength services to new types of actors (e.g. verticals).
Thus, it is anticipated that it will become increasingly important to be able to flexibly provision and optimise usage of the fixed access infrastructure resources. In today's optical access systems, use of Wavelength Division Multiplexing, WDM, is limited due to the cost of the interfaces. However, ongoing efforts in realising low cost tunable lasers to enable colourless transceivers and with the introduction of WDM technology in the next-generation optical access systems (through Time Wavelength Division Multiplexing-PON, TWDM-PON), WDM is expected to become increasingly important for the access. To further introduce wavelength switching requires devices such as Wavelength Selective Switches, WSSs and Reconfigurable Optical Add Drop Multiplexers, ROADMs, which currently are costly for the access segment. However, a recently proposed Multi Directional WSS, MD-WSS, which enables more efficient use of the wavelength routing capabilities of the WSS could be used to reduce the barrier for wavelength switching in the access. In a longer time horizon, integrated photonics promises low cost devices for wavelength switching that could be applicable for the access segment.
Current access networks are inflexible in terms of multi-service/multi-operator support. Certain deployment models (combined fixed access and fronthaul over the same fibre infrastructure) are either not supported or require large effort in terms of network planning and service provisioning (e.g. installing fixed co-existence filters). Current optical access networks provide limited flexibility for optimising network resource utilisation (e.g. enabling optical Line Terminal, OLT, sleep modes). Also, migration and upgrade often results in service disruption and require careful planning (due to limited possibilities for re-routing access traffic during service windows).